1. Field of the Invention
This invention relates to a rotation detector. In particular, the present invention can be favorably applied to a rotary encoder which detects information on the rotation of a rotating member having an optical scale including transparent gratings obtained by providing, for example, a plurality of projections and recesses periodically on, or on a portion of, the outer circumferential surface or the inner circumferential surface of a cylindrical member by projecting a light beam onto the rotating member and utilizing light beams emanating from the optical scale.
2. Description of the Related Art
Photoelectric rotary encoders have been utilized as means for detecting the rotational speed or the amount of variations in the rotational speed of a rotating mechanism, such as a capstan motor, a rotating drum or the like, used in a computer unit, such as a floppy-disk drive or the like, a business machine, such as a printer or the like, an NC (numerical control) machine tool, a VCR (video cassette recorder) or the like.
FIGS. 1 and 2 are a cross-sectional view and a plan view of a principal portion of a rotary encoder which utilizes so-called Talbot's interference, respectively, proposed in, for example, Japanese Patent Laid-open Application (Kokai) No. 3-197818 (1991). In addition, encoders which utilize Talbot's interference proposed in Japanese Patent Laid-open Application (Kokai) Nos. 61-10716 (1986) and 1-176914 (1989) are known.
In FIGS. 1 and 2, semiconductor laser 1 emits a coherent light beam having a wavelength .lambda.. Collimating lens system 2 converts the divergent light beam from semiconductor laser 1 into a parallel light beam. Semiconductor laser 1 and collimating lens system 2 constitute light irradiating means LR. Optical scale 3 includes transparent grating portions obtained by providing a plurality of V-grooves periodically on the inner circumferential surface of cylindrical member 3c, and rotates in the direction of the arrow around rotation shaft 6a.
Optical scale 3 is made of a transparent optical material, and constitutes a portion of rotating member 6. Three photodetectors 4a, 4b and 4c (FIG. 2) constituting light sensing means 4 are disposed at positions facing light irradiating means LR across optical scale 3. The output of each photodetector is connected to signal processing circuit 5. Signal processing circuit 5 includes pulse counting circuitry, circuitry for determining the direction of the rotation of optical scale 3, circuitry for signal interpolation processing, and the like.
The rotatry encoder shown in FIGS. 1 and 2 performs light modulation (deflection) by projecting the light beam from light irradiating means LR onto first scale 3a in one region of optical scale 3, and projecting the light beam modulated (diffracted) by optical scale 3b onto second scale 3b on another region of optical scale 3. A plurality of (three) light beams emanated from second scale 3b of optical scale 3 are sensed by light sensing means 4. Information regarding the rotation of optical scale 3 is detected utilizing output signals from light sensing means 4.